ANKARA (Reuters) - Tayyip Erdogan will cement his position as modern Turkey's most powerful leader when he is sworn in as president on Thursday, advancing his drive to reshape the country but heralding what critics fear will be an increasingly authoritarian rule.

In a final speech to supporters of his AK Party on Wednesday he spoke of his move from the prime minister's office to the presidential palace as the birth of a new Turkey. But he vowed that the AK's mission to elevate the country as a major regional power would go on unchanged after he left party politics.

He will be sworn in as president at around 0700 GMT.

Erdogan's victory in Turkey's first popular presidential election this month caps more than decade as prime minister in which the economy has tripled in dollar terms and the country has carved out a growing, though often controversial, role in the politics of the conflict-torn Middle East.

Opponents warn his ambition to establish an executive presidential system will concentrate too much power in the hands of a leader with autocratic instincts and lead the EU candidate country ever further from the secular ideals of the republic's founder Mustafa Kemal Ataturk.

Erdogan's rhetoric has long played on the divisions between his supporters among Turkey's pious conservatives and a Western-facing, largely secular class suspicious of his Islamic ideals. In his farewell party address, he tried to strike a more conciliatory note.

"Whether they love us or not, I reach out my hand from here to every one of the 77 million people," he said. "We understand your lifestyles, your values... We want you to understand the bans, restrictions and threats we overcame to get here today."

His combative nature was still in evidence, however, as he vowed to fight on against the "treachery" of his ally-turned-foe U.S.-based cleric Fethullah Gulen. Erdogan accuses Gulen of orchestrating a corruption scandal targeting the government through a network of followers in the police and judiciary.

That power struggle, along with anti-government demonstrations last summer, have made for one of Erdogan's most difficult years in office. But he has bounced back with success in both local and presidential elections this year.

CHALLENGES AHEAD

After being sworn in, Erdogan will lay a wreath at Ataturk's mausoleum before a ceremony at the presidential palace.

Senior representatives of some 90 countries from Asia, Africa, the Middle East and Europe will attend, including the emir of Qatar, the president of Ukraine, and Iran's foreign minister, according to Erdogan's office, although no major Western heads of state are expected.

Erdogan now faces the challenge of retaining his grip on the country in a role which has so far been largely ceremonial and which requires that he sever his links with the ruling AK Party, 12 years after it first came to power.

After assuming the presidency Erdogan will ask outgoing Foreign Minister Ahmet Davutoglu, who took over from him as AK Party leader on Wednesday, to form a new government. A new cabinet is set to be announced on Friday.

The current economic team, including Deputy Prime Minister Ali Babacan and Finance Minister Mehmet Simsek, is expected to remain largely intact, while intelligence chief Hakan Fidan, a close Erdogan confidante, and EU minister Mevlut Cavusoglu are leading contenders for foreign minister.

Erdogan aide Yalcin Akdogan was also expected to take up a position in cabinet, possibly as a deputy prime minister, while AK deputy chairman Mustafa Sentop is seen as a candidate for justice minister, senior officials have said.

The AK party must win a stronger majority in parliament in a general election due by next June if Erdogan is to secure his ambition of changing the constitution and establishing an executive presidency.